Drives (also known as “drive units”) are used to power and control electric motors. Many drives comprise an inverter, which produces an AC output that is sent to the motor. The inverter comprises a plurality of semiconductor devices (e.g. transistors and diodes). When the drive is used to provide a high power output and/or a low output frequency, there is a risk of one or more of the semiconductor devices overheating due to power loss in the devices, thus causing the drive to malfunction. In order to ensure reliable usage of a drive, such overheating is to be avoided.
The simplest way to avoid overheating is to limit the maximum power output of the drive to a conservative level. However, this is likely to overcompensate and result in a drive that is unnecessarily limited in its power output. There is a desire to determine the temperatures of the semiconductor devices more accurately, so that the highest possible power output can be achieved while ensuring the safe and reliable operation of the drive.
Thermal models have been developed that estimate the junction temperature of each of the semiconductor devices, which can then be limited below the maximum value. Many inverters use a single package that contains multiple IGBTs and diodes, totaling N devices. In this type of package, a power loss in one device influences the temperature of that device and also every other device in the package due to its proximity to the other devices resulting in heat transfer to the other devices.
The junction temperature (Tj) of each of the semiconductor devices cannot be measured directly, therefore the temperature rise between the junction of one of the devices device and a measured reference temperature (Tref) can be estimated using a real-time thermal model in the time domain. To calculate the temperature between the junction of each device and the measured reference temperature, a thermal impedance matrix can be used with the instantaneous power loss in each device, which is proportional to the phase current. A thermal impedance matrix includes the self impedance of each device and the mutual thermal impedances of the other devices with respect to each device in the inverter. For a three phase inverter with six IGBTs and six diodes, the matrix includes 144 elements. The main limitation that prevents this method being freely implemented in a drive is the processor resources required to implement the full thermal impedance matrix. To implement this type of model a considerable number of calculations are to be performed by the processor during each sample period, and since these are carried out in the time domain, the sample rate is to be high enough to prevent aliasing when the inverter is operating at high output frequencies. To achieve this, a significant computational resource is required, which as of yet this is not available in a commercial drive control system.
Attempts have been made to simplify the full thermal impedance matrix, such as by reducing the number of elements in the matrix that are calculated, but these typically result in significant temperature errors, resulting in an underpowered or unreliable drive. Such simplifications have operated in the time domain. It has been appreciated by the inventors that the possible simplifications are limited in the time domain because the peak temperature is proportional to the current in the device. At a non-zero output frequency, the current in one of the output phases is sinusoidal and each device will only conduct for half of the cycle. If the frequency is reduced to zero, when there is no current flow in a device the power loss and the temperature rise due to the self thermal impedance will be zero. Therefore, it is not possible to simplify the model by estimating the temperature of a single device as this will not protect the inverter in this condition.
It has been established by the inventors that, in the frequency domain, the peak steady-state temperature of a device can be determined from the harmonics of the temperature response, which allows the thermal impedance matrix to be reduced to as few as one or two devices. This is a significant advantage when compared to the implementation in the time domain and allows the thermal model to be implemented using the available processor resources of current drives. In a steady-state operating condition, this type of model will not be affected by aliasing and can be implemented using a moderate sample frequency. Furthermore, protection all of the devices in the inverter, can be achieved by calculating only the peak temperature of the hottest IGBT and diode. In the model implemented in the frequency domain, the current magnitude and output frequency are used instead of the three instantaneous phase currents.
In addition, methods have been developed to calculate the peak temperature from the harmonics, select the hottest device for a given set of conditions and combine the self thermal impedances to further simplify the model.